1. Field of the Invention
Generally, the field of the present invention involves temperature measurement of high power light sources. More particularly, the present invention relates to temperature measurement with pyrometers for systems utilizing high power light sources.
2. Background
For many material processing applications, processing techniques utilizing high power light sources, such as laser light sources, are preferred over other conventional processing techniques, such as mechanical or chemical ones. High power light sources can achieve the tight precision and accuracy requirements for the fabrication of both small and large structures, while maintaining very high wall-plug efficiency. High power systems typically include those producing 30 W to multiple kW or even higher. During operation, the work surface is typically monitored for closed or open-loop control of material processing variables or more generally to verify successful operation of the optical processing system. Different techniques may be used to monitor work surface temperature, such as contact measurement through thermocouples disposed in proximity to the surface or indirectly through remote sensing.
In one conventional remote sensing example, in order to monitor and maintain the process parameters of the light source at the work surface, one or more optical pyrometers are typically positioned in relation to the work surface and light source and aligned with the spot of the light source at the work surface. The pyrometer receives light emitted via thermal radiation from the material at the work surface piece and achieves a modicum of accuracy by overlapping the spot of the light source at the work surface with an acceptance aperture of the pyrometer. The pyrometer then typically filters incoming light so that only a particular wavelength is received and measured in order to determine parameters of the work surface or various layers thereof, such as surface temperatures. The pyrometer can be disposed away from the work surface, including behind one or more transparent windows, for contactless temperature measurement. Thus, remote sensing is generally preferred for improved accuracy, simplification of processing, and for lack of interference with the optical material processing assembly.
While useful for detecting uniform temperature profiles, such as oven-heated material work surfaces, problems arise when remote sensing involves incident material processing light at higher powers or tighter tolerance requirements. Careful alignment is required between the spot of the pyrometer signal and the spot of the processing beam. Furthermore, because of short working distances between the light source process optic and the work surface, conventional pyrometer optics have difficulty in imaging to the same spot as the focused high power laser light sources. Accordingly, the spot gathered by the pyrometer at the work surface is often a different size from the spot of the light source, resulting in inaccuracy in the temperature measurement. Also, the light source process optic may be capable of providing a variable focus or other variable parameters, altering the size or shape of the beam. Conventional pyrometers must be adjusted or recalibrated accordingly. For some applications a non-circular processing spot is used, such as a line, rectangle, square, hexagon, etc. The overlap between such spots and the typical circular shape of the pyrometer optic and associated beam are generally poor, leading to additional inaccuracies. Thus, there remains a need for a pyrometer temperature measurement process for use in conjunction with high powered light sources, without any of the attendant drawbacks.